>This goes back, I think, to an earlier issue, which is that Olympia>lacks a mechanism for player-player control. I can build a castle and>garrison a continent, but I can't prevent any random noble from>landing a ship, getting out, recruiting my peasants into his army,>sawing down trees in my forests, etc.

Sure you can. All resource collection is reported to garrisons, and
then you can take your army and kill whoever's doing it, or instruct
your garrisons to attack them. They can get only one turn of
collection in before you can react. That's not enough time to threaten
your empire, so the existing mechanics seem to do the job.